Acid-impregnated activated carbon and methods of forming and using the same

ABSTRACT

An acid-impregnated activated carbon matrix is formed from a carbonaceous material by the addition of a mineral acid, and may be used to chemisorb ammonia from a gas stream. The ammonia reacts with the acid to form a fertilizer salt. The spent matrix may be used as a fertilizer, or the fertilizer salt may be elutriated from the matrix.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. ProvisionalApplication No. 60/823,347 filed on Aug. 23, 2006 entitled “Preparationof Ammonia-Chemisorbent Carbonaceous Material Using Liquid Acid”, thecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a composition comprising an activatedcarbon matrix impregnated with a mineral acid and methods of producingand using the same.

BACKGROUND OF THE INVENTION

Ammonia is an important chemical in industry and agriculture. It is usedin the manufacture of many polymers and textiles, as well as being theessential foundation of nitrogen fertilizers.

Ammonia found in air or water may originate from the decomposition ofurea, proteins, and other nitrogenous organic substances, or from theaccidental escape of ammonia during its use in industry or agriculture.Ammonia in air is toxic to humans and animals at concentrations of 25 to500 parts per million, depending on the acceptable exposure time. At anyconcentration, ammonia in air combines with acidic components, such assulphur dioxide, to form particulate matter less than 2.5 um diameter(PM_(2.5)), which is a particularly noxious pollutant that can penetratedeep inside the human respiratory tract. In addition, airborne ammoniacauses corrosion of metal structures and is considered to be a majorcontributor to odour problems.

Ammonia is highly soluble in water, where it can cause fish mortality athigh concentrations and contribute to eutrophication and a depletion ofoxygen by stimulating the growth of algal populations.

Ammonia may be removed from air by several methods. First, and mostinexpensively, ammonia-laden air is diluted with air of low ammoniaconcentrations so that acceptable levels are achieved. However, this“dilution” approach distributes ammonia over a wider area and thuscontributes to the formation of PM_(2.5). In confined livestockoperations, where toxic levels of ammonia build up as a result of animalurine deposition, inside air is expelled and outside air is brought inas “make-up air”. However, under cold climate conditions, the removal ofammonia-laden air requires heating of replacement air to keep eventemperatures inside the barn.

Another option is to remove ammonia from air by bubbling it throughwater, thereby trapping the ammonia as aqueous ammonia and ammonium ion(NH₄ ⁺). However, as ammonium levels increase, the pH of the waterincreases and ammonia is released into the air again. Furthermore,dilute ammoniated water is not valuable and must be disposed of as well.A third option, and perhaps the most common of all, is to bubbleammonia-laden air through mineral acids, such as sulphuric orhydrochloric or nitric acid. The ammonia is converted to the equivalentsalt (ammonium sulphate, ammonium chloride or ammonium nitrate). Thedisadvantages of the third option are: (a) considerable back pressuredevelops as a result of bubbling air through liquids and (b) the saltsthat are formed are mixed with the liquid acid and are difficult toseparate, thereby limiting the usefulness of the by-products.

Another option is to reduce ammonia to nitrogen gas (N₂) byelectrochemical treatment, however, this method suffers from highoperating costs and the requirement for complex processing equipment.

If ammonia must be removed from water, such as from wastewater that willbe reintroduced to natural water bodies, the ammonia is stripped fromthe water into air, where it becomes an air-removal problem again.Therefore, all of the technologies discussed above for removing ammoniafrom air are equally applicable to treating ammonia in water.

Therefore, there is a need in the art for a activated carbon matrix thatremoves ammonia from air, which may mitigate some or all of thedifficulties found in the prior art.

SUMMARY OF THE INVENTION

The present invention is directed to a novel composition comprisingacid-impregnated activated carbon, which may be produced by convertingcarbonaceous material into an activated carbon matrix while infusingacid within the activated carbon matrix. In addition, the presentinvention may include a method of using acid-impregnated activatedcarbon to remove ammonia from gas streams, and to a method for producinga fertilizer material from the spent media. Further, the presentinvention may include a method for converting the acid-impregnatedactivated carbon matrix and the fertilizer salt impregnated by-productinto activated carbon.

Thus, in one aspect, the invention may comprise a solid compositioncomprising an activated carbon matrix impregnated with a mineral acid,which may be useful for chemisorbing ammonia. In one embodiment, themineral acid comprises one of sulphuric acid, hydrochloric acid,phosphoric acid or nitric acid. The solid composition preferably has asurface area of at least about 10 m²/gram, more preferably at leastabout 30 m²/gram, and most preferably at least about 500 m²/gram.

In one embodiment, the activated carbon matrix is formed from acarbonaceous material by the addition of the mineral acid to thecarbonaceous material, wherein the activated carbon matrix has a surfacearea of at least about 5 times that of the carbonaceous material, andpreferably about 10 times, and more preferably about 100 times, and mostpreferably about 300 times the surface area of the carbonaceousmaterial. The carbonaceous material comprises a biomass materialcomprising once living organisms or any materials formed from onceliving organisms, for example, wood, animal waste product, or peat moss.

In another aspect, the invention may comprise a method for producing anactivated carbon matrix impregnated with mineral acid comprising thesteps of:

-   -   (a) if necessary, drying carbonaceous material to a suitable        moisture content;    -   (b) grinding carbonaceous material to a suitable particle size        range; and    -   (c) applying a mineral acid to the carbonaceous material while        mixing both components.

In one embodiment, the carbonaceous material may comprise wood, animalwaste product, or peat moss. The carbonaceous material may be pelletizedprior to applying a mineral acid, or the activated carbon matriximpregnated with a mineral acid may be pelletized. In one embodiment,the activated carbon matrix impregnated with a mineral acid may beelutriated with water to wash out fertilizer salt after ammoniachemisorption.

In another aspect, the invention may comprise a method of chemisorbingammonia from a gas stream comprising the step of passing the gas streamover or through an activated carbon matrix impregnated with a mineralacid. In one embodiment, the method may comprise the steps of:

-   -   (a) placing an activated carbon matrix impregnated with a        mineral acid in a reactor; and    -   (b) flowing an ammonia-containing gas through the reactor.

In one embodiment, the activated carbon matrix impregnated with amineral acid is disturbed during gas flow and may be in pelletized orgranular form.

In another aspect, the invention may comprise a method of convertingacid-impregnated activated carbon matrix into a fertilizer productcomprising the steps of:

-   -   1. converting acid in the activated carbon matrix to its        corresponding salt by exposing the spent activated carbon matrix        to ammonia gas; and    -   2. screening the activated carbon matrix to a desired particle        size range or pelletizing the activated carbon matrix to achieve        a desired particle size range.        In one embodiment, the method further comprises the step of        elutriating the fertilizer product from the activated carbon        matrix, leaving behind activated carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying simplified, diagrammatic,not-to-scale drawings. In the drawings:

FIG. 1 shows a graph showing the relationship of superficial velocity ofthe flowing gas to the required bed depth of activated carbon matrix tomaintain a desired minimum pressure drop.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a solid porous media produced fromcarbonaceous materials that are impregnated with acid, and an apparatusfor removing ammonia from a gas stream by flowing the gas through thesolid porous media impregnated with acid, and a composition that is theby-product of reacting ammonia with the acid impregnated in theactivated carbon matrix. When describing the present invention, thefollowing terms have the following meanings, unless indicated otherwise.All terms not defined herein have their common art, recognized meanings.

To the extent that the following description is of a specific embodimentor a particular use of the invention, it is intended to be illustrativeonly, and not limiting of the claimed invention. The followingdescription is intended to cover all alternatives, modifications andequivalents that are included in the spirit and scope of the invention,as defined in the appended claims.

“Carbonaceous material” shall mean any biomass material, which includesrecently or once living biological material such as plants, animals,algae, or micro-organisms, or any materials or residues formed from onceliving organisms. Carbonaceous materials may include, withoutlimitation, wood and other lignocellulosic material, animal waste orbyproducts such as digested or composted animal manure, agriculturalbyproducts, peat moss, straw, municipal solid waste, bedding materialscontaining manure, nut shells, coconut coir, and fossil fuels and fossilfuel byproducts such as coal and petroleum coke.

“Liquid acid” shall mean any inorganic acid including, but not limitedto, sulphuric, phosphoric, nitric, or hydrochloric acid.

“Activated carbon” shall mean a solid microporous material with highsurface area comprised primarily of elemental carbon and containingsmall amounts of other elements originally found in the carbonaceousmaterials from which the activated carbon was formed, which may includebut are not limited to such elements as oxygen, hydrogen, nitrogen,sulphur, silicon, aluminum, iron, calcium, magnesium, sodium, andpotassium.

“Activated carbon matrix” shall mean activated carbon in a solid formsufficiently porous to allow passage of gas through its interior spaces.

“Gas” shall mean any substance or combination of substances that existsin a gaseous state at standard temperature and pressure.

“Chemisorption” shall mean the attachment or adsorption of a gasmolecule onto a solid or liquid surface and any reactions that mightensue between the gas molecule and the solid or liquid.

The inventors have found that carbonaceous materials will react withliquid acid to form an activated carbon matrix impregnated with theacid. This reaction may occur under ambient conditions.

In general terms, an acid-impregnated activated carbon matrix may beformed by:

-   -   1. if necessary, adjusting the moisture content of a        carbonaceous material to the desired level;    -   2. adjusting the particle size of the carbonaceous material to        the desired range;    -   3. applying liquid acid to the carbonaceous material; and    -   4. mixing the carbonaceous material and liquid acid until the        chemical reaction is complete.

The carbonaceous material may comprise any suitable biomass material,including wood and other lignocellulosic material, animal waste orbyproducts such as digested or composted animal manure, peat moss,straw, municipal solid waste, bedding materials containing manure, nutshells, coconut coir, coal and petroleum coke. Wood chips or shavingsare a particularly preferred carbonaceous material.

The moisture content of the carbonaceous material depends on thefeedstock and the particle size, and may have a range of about zero to50% on a wet mass basis, preferably about 5 to 35% and more preferablyabout 15 to 25%. The carbonaceous material may be dried if the moisturecontent is higher than the desired level, or water may be added to thecarbonaceous material to bring up the moisture level.

The carbonaceous material may be processed into particles of anappropriate size, depending on the intended application and thefeedstock, by any suitable method, including for example, chopping,grinding, cutting or otherwise reducing the particle size. Additionally,if the feedstock consists of very small particles, the particles maybeagglomerated to create larger particles of a suitable size. The particlesize of the carbonaceous material may have an average range of about 0.1mm to 10 mm, preferably about 1 to 5 mm and more preferably about 3 mm.

The liquid acid may be any suitable mineral acid, such as sulphuric,phosphoric, hydrochloric, or nitric acid. The choice of acid will ofcourse change the salt formed if the acid reacts with a chemisorbedmolecule. Thus, if the material is being used to remove ammonia from agas stream, then the use of sulphuric acid will result in the formationof ammonium sulphate.

The concentration of liquid acid used depends on the moisture content ofthe carbonaceous material, lower concentrations are suitable for lowermoisture content, and may have a range of about 20 to 100%, preferablyabout 75 to 100% and more preferably 100% (where 100% is theconcentrated form of the acid). The amount of liquid acid used dependsin part on the particle size of the carbonaceous material and theconcentration of the acid used, and may have a range of about 1 partacid to 1 part carbonaceous material (by weight) for smaller particles,to 10 parts acid to 1 part carbonaceous material for the largest (about10 mm) particles. Preferably, the ratio of acid to carbonaceous materialis about 2:1 to 5:1 and more preferably about 4:1 (by weight).

The carbonaceous material and the liquid acid are mixed until thereaction is substantially complete, which length of time depends on themoisture content, particle size, acid concentration and acid/feedstockratio, but is typically between about 2 to 35 minutes, preferably about5 to 25 minutes and more preferably about 15 minutes. In one embodiment,completion of the reaction may be monitored by temperature. As thereaction starts, the temperature typically rises to reach a maximum andfalls as the reaction completes.

In one embodiment, the liquid acid is sprayed on the carbonaceousmaterial as mixing proceeds. In another embodiment, the carbonaceousmaterial is formed into pellets and then the liquid acid is applied tothe pelletized form of carbonaceous material.

The acid converts the carbonaceous material into an activated carbonmatrix, and excess acid impregnates itself onto the activated carbonmatrix. Even though there are large amounts of acid impregnated in thecarbon matrix, the product looks and behaves as a solid material. Asolid matrix containing large amounts of a strong acid is scientificallyand commercially important because gas can be flowed through porousactivated carbon matrix more efficiently and inexpensively than throughan equivalent amount of liquid.

In one embodiment, the conversion of the carbonaceous material toactivated carbon, and the impregnation of acid, takes place in one step.Furthermore, the acid-impregnated activated carbon matrix does notrequire further processing prior to use as a chemisorbent. Thus, no heattreatment, washing or neutralization step, or subsequent gas sulfonationstep is required or desired.

As a result, the acid-impregnated carbon matrix may be used as achemisorbent material because of its microporosity and large surfacearea. Hence, any basic constituent in a gas which is flowed through thematerial can be more efficiently removed and converted into a solidby-product.

In one embodiment, the material may be used to remove ammonia from a gasstream. Ammonia reacts with inorganic acids to form the correspondingammonium salt and will be retained by the solid material as the gaspasses through.

A gas stream containing ammonia may be routed through a sealed reactionchamber comprising the acid-impregnated activated carbon matrix, eitherin solid, granular or pelletized form. The activated carbon matrix maycomprise a fixed bed or may be disturbed by gas flow or by mechanicalmeans, such as with a fluidized bed, or a pseudofluidized bed.Preferably, means are provided to periodically replenish or replace theactivated carbon matrix.

The ammonia is chemisorbed by the acid-impregnated activated carbonmatrix and converted to a fertilizer salt with little residual acidityand only small amounts of carbon and other elements. Thus, the spentactivated carbon matrix is a useful source of selected nutrients foragriculture and horticultural applications. As such, the expense ofammonia removal is reduced and a value-added by-product is created.

The spent activated carbon matrix may be pelletized using conventionalmethods to form fertilizer pellets or otherwise processed into a usefulagricultural or horticultural form. If pelletized or processed ingranular form, the pellets may provide a slow-release mechanism for theammonium salt fertilizer.

In one embodiment, the ammonium salt, such as ammonium sulphate, iselutriated from the activated carbon matrix with water. The ammoniumsulphate solution can then be concentrated and formed as a fertilizer,leaving the activated carbon matrix.

EXAMPLES

The following examples are intended to illustrate but not limit theclaimed invention.

Concentrated sulphuric acid was added to carbonaceous material in weightratios varying from about 2.5:1 to about 4.5:1. The temperature of thematerial was monitored, and the final acid content of the material wasrecorded. The results are shown in Table 1 below.

TABLE 1 Final acid content and maximum temperature reached in activatedcarbon matrix after adding sulphuric acid to carbon source Max. Finalacid Ratio of Acid to temperature content Carbon Source (° C.) (%) 2.5166.0 71.0 3.0 155.0 74.0 3.5 125.0 80.2 4.0 96.5 78.9 4.5 86.0 82.6

It can also be seen from Table 1 that liquid acid not only transformsthe carbonaceous material to activated carbon but results in theimpregnation of acid in activated the carbon matrix. Depending on theratio of liquid acid to carbonaceous material, as much as 82% by weightof the resulting activated carbon matrix is comprised of acid.

Furthermore, Table 1 shows that the maximum temperature of the reactiondecreases as the ratio of acid to carbonaceous material increases.Although not shown in Table 1, none of the trials resulted in more thanfive percent loss in mass balance, that is, the sum of the loss ofcarbonaceous material and acid during the reaction to produce the solidproduct did not exceed five percent.

The extent of the reaction of several carbonaceous materials withsulphuric acid was quantified. The transformation of severalcarbonaceous materials into porous activated carbon matrix comprisingactivated carbon was demonstrated by a large change in surface area.

TABLE 2 Effect on surface area of adding concentrated sulphuric acid(2.5 parts) to carbonaceous materials (1 part). Surface Area BeforeAfter Treatment Treatment Carbonaceous Material m²/g Wood shavings 2.07630.89 Animal biosolids⁽¹⁾ 3.51 34.14 Peat moss 2.06 10.34 ⁽¹⁾Derivedfrom anaerobic digestion

Reacting a carbonaceous material with a liquid acid leads to a largeincrease in surface area of the carbon matrix, especially of wood. Thesurface area of wood shavings prior to the reaction was approximatelytwo square meters per gram; after the reaction the surface areaincreased to more than six hundred square meters per gram. Thisrepresents approximately a three-hundred fold increase in surface area.It should be noted that biosolids originating from the anaerobicdigestion of cattle manure showed approximately a ten-fold increase insurface area as a consequence of treatment with sulphuric acid, whilecommercial peat moss showed approximately a five-fold increase insurface area from the same treatment.

It has been surprisingly found that any concentration of ammonia in agas stream will be completely and rapidly chemisorbed by theacid-impregnated activated carbon matrix.

TABLE 3 Effect of carrier gas characteristics and NH₃ concentration ofinlet gas on NH₃ adsorption Properties of Carrier Gas Carbonaceous BedSuperficial NH₃ Concentration source of Depth Velocity Temp R.H.⁽¹⁾Moisture Inlet Outlet adsorbent (cm) (cm/s) (° C.) (%) (%, v/v) (ppm)Wood shavings 3.2 46.7 22 45 1.18 95 <1 Wood shavings 1.4 9.6 22 1002.58 1,994 <1 Wood shavings 1.0 11 60 100 20.32 1,767 <1 Wood shavings9.0 3.1 21 0 0.00 80,200 <1 Wood shavings 9.0 3.1 21 100 2.43 80,200 <1Wood shavings 7.0 7.9 23 0 0.00 150,000 <1 Animal biosolids⁽²⁾ 1.3 9.723 100 2.74 1,986 <1 ⁽¹⁾Relative Humidity ⁽²⁾Derived from anaerobicdigestion

The results tabulated in Table 3 shows that ammonia in a gas stream,ranging from ninety five parts per million by volume to one hundredfifty thousand parts per million by volume, is chemisorbed by theactivated carbon matrix so that outlet concentrations of ammonia areless than one part per million. Furthermore, Table 3 shows that varyingtemperature or relative humidity of the gas does not affect ammoniachemisorption, provided that a significant decrease in temperature doesnot occur.

In order to determine critical response variables, testing was conductedto determine the minimum bed depth and reaction time required to adsorb100% of ammonia present in a gas stream.

TABLE 4 Effect of activated carbon matrix characteristics and gastemperature on critical response variables⁽¹⁾. Critical Response GasCharacteristics Variables⁽¹⁾ Carbonaceous NH₃ Inlet Superficial RelativeBed Reaction Source of Concentration Velocity Temperature Humidity DepthTime Adsorbent (ppmv) (cm/s) (° C.) (%) (mm) (msec) Wood shavings 19949.6 22 100 7 75 Wood shavings 1767 11 60 100 7 62 Wood shavings 1991 1723 100 10 61 Animal 1986 9.7 23 100 11 109 biosolids⁽²⁾ ⁽¹⁾Criticalresponse variables are the minimum parameter values required to adsorb100% of the NH₃ ⁽²⁾Derived from anaerobic digestion

Table 4 shows that only seven to eleven millimetres of acid-impregnatedactivated carbon matrix is needed to quickly (sixty one to one hundredand nine milliseconds) chemisorb all ammonia (approximately two thousandparts per million by volume) from a gas flowing at ten to seventeencentimetres per second. Our conclusion is that ammonia chemisorption isvery rapid and needs very little exposure to the mass ofacid-impregnated activated carbon matrix to be completely removed. Table4 shows that high gas temperatures (60° Celsius) do not affect theretention time needed to chemisorb ammonia, as long as the gas does notdrop in temperature as it passes through the activated carbon matrix(all gas streams were saturated with moisture at their respectivetemperatures). It is also noteworthy that Table 3 and Table 4 show thatthe source of the carbonaceous material, whether it originates from woodshavings or biosolids from cattle manure, does not significantly affectthe required retention time for ammonia chemisorption.

We determined by testing that the acid-impregnated activated carbonmatrix, even when it has been converted to its fertilizer salt, willfacilitate the flow of gas with minimum pressure drop even, and evenwhen flow rates are high. The graph shown in FIG. 1 shows that fornon-pelletized, acid-impregnated activated carbon matrix that hasalready been converted to its fertilizer salt by the chemisorption ofammonia, the pressure drop does not exceed one and a half kilopascalseven with flow rates of eighty centimetres per second through tencentimetres of activated carbon matrix. It is also apparent in FIG. 1that as the flow rate decreases, the depth of the activated carbonmatrix bed can increase exponentially without causing a pressure drop ofmore than one and a half kiloPascals. Also, testing determined that, atthe same gas flow rate, measured as superficial velocity, a disturbedbed—one that is periodically vibrated—causes less pressure drop than a“fixed” bed, that is, one that is not disturbed during testing.

The amount of ammonia adsorbed by acid-impregnated activated carbonmatrix was measured by the ratio of ammonia adsorbed per unit mass ofthe activated carbon matrix. Table 6 shows that the acid-impregnatedactivated carbon matrix adsorbs between two hundred and two hundred andtwenty three milligrams of ammonia per gram of activated carbon matrix,representing twenty to twenty three percent of ammonia by weight.

TABLE 6 Total NH₃ adsorbed (per gram activated carbon matrix) and bulkdensity of ’spent’ acid-impregnated activated carbon matrix in relationto original particle size. Particle size NH₃ Adsorbed Bulk Density (mm)(mg/g) (kg/m³⁾ <0.5 200 513 0.5-1.0 230 614 1.0-1.7 220 628 1.7-2.0 220623 2.0-2.8 220 547 2.8-3.4 230 636 3.4-4.0 220 610 >4.0 230 680

Table 6 also shows that the bulk density of the activated carbon matrixafter chemiadsorption of ammonia increases to approximately five hundredto seven hundred kilograms per cubic meter.

Testing was conducted to determine the chemical composition ofacid-impregnated activated carbon matrix after chemisorption of ammonia.The fully converted acid-impregnated media is termed “spent” activatedcarbon matrix. Table 7 shows the chemical composition of the spentactivated carbon matrix after full chemisorption of ammonia has beencompleted. It can be seen from FIG. 7 that the fertilizer salt compriseseighty four percent by weight of the spent activated carbon matrix aftercompleting ammonia chemisorption.

TABLE 7 Components in acid-impregnated activated carbon matrix after NH₃adsorption completed. Amount Components (%) Ammonium 84.0 sulphateElemental composition⁽¹⁾ Nitrogen 18.0 Sulphur 11.2 Carbon 5.9 Oxygen40.0 Other 4.3 Residual acid 0.6 ⁽¹⁾Includes elements in ammoniumsulphate and residues from adsorbent matrix.

Furthermore, Table 7 shows that only six tenths of one percent of theoriginal acid remains in the spent activated carbon matrix. Theelemental composition of the spent activated carbon matrix is consistentwith the large proportion of fertilizer salt, which is ammonium sulphatein the case of the experiment giving rise to the data presented in Table7. The carbon content remaining from the original wood shavings or othercarbonaceous materials is less than six percent by weight.

1. A method for producing a porous carbon matrix impregnated withmineral acid directly from a carbonaceous biomass material, comprisingthe step of applying a mineral acid to the carbonaceous biomass materialwhile mixing both components, in one step, without the addition of heatsubsequent to the application of acid, and without further washing orneutralization, thereby leaving the mineral acid impregnated in theporous carbon matrix.
 2. The method of claim 1 further comprising thestep of drying or wetting the carbonaceous biomass material to asuitable moisture content prior to acid application.
 3. The method ofclaim 1 further comprising the step of grinding the carbonaceous biomassmaterial to a suitable particle size range prior to acid application. 4.The method of claim 1 wherein the carbonaceous biomass materialcomprises wood, digested or composted animal manure, peat moss, straw,municipal solid waste, bedding materials containing manure, nut shells,or coconut coir.
 5. The method of claim 1 further comprising the step ofpelletizing the carbonaceous material prior to applying a mineral acid.6. The method of claim 1 further comprising the step of pelletizing orgranulating the carbon matrix impregnated with a mineral acid.
 7. Themethod of claim 1 wherein the mineral acid comprises sulphuric, nitric,phosphoric or hydrochloric acid.
 8. The method of claim 7 wherein themineral acid comprises concentrated sulphuric acid.